Soil vapor extraction (SVE) is an in-situ remediation method used for cleaning up soil and removing vapor from below the surface. Vapor extraction occurs with the aim of treatment above the ground. Cleansing of vapor recovered from the subsurface involves recovering or destroying the pollutants. When contaminant chemicals dry out, they form the gases called vapors. SVE is a safe and cost-friendly technology enabling the effective removal of volatile organic compounds (VOC) and site decontamination. The most common VOCs are petroleum and chlorinated compounds. SVE helps clean ground under buildings with little or no disruption to everyday life, and there are no risks to human health and the environment. SVE combines with other in-situ treatment technologies such as soil fracturing, air sparging, and pump-and-treat.
The effectiveness of SVE primarily depends on the type of contamination. Typically, soil vapor extraction is suitable for cleaning up and removing volatile organic compounds. These are chemicals with a low boiling point. Additionally, they have a high vapor pressure. SVE is the most effective with highly volatile organic compounds because these liquids have the lowest boiling point causing them to evaporate into the air.
Contaminant concentration is another factor affecting the SVE effectiveness. The higher the concentration of volatile organic compounds, the longer the process of SVE. The more contaminants in the soil equal complex extraction with the SVE system. Hence, depending on contaminant concentration, the effectiveness of SVE can vary.
The larger and deeper the contamination site means a more complex soil vapor extraction process. In addition, the soil type and its permeability are also decisive factors. SVE process is the most suitable for sandy soils because its high permeability allows better airflow. In practice, SVE gives the best results in sites where the groundwater table is approximately 10 feet (or more) below the surface. That is because soil moisture and the occurrence of upwelling prevent the soil vapor flow. When that happens, SVE experts use a pump to lower the water table level and increase the vadose zone.
The design of the SVE system largely determines its effectiveness. To create a soil vapor extraction system, operators design a system that includes installing wells in the contaminated area. The depth of the wells is slightly above the highest water table elevation. Connecting wells to a blower creates a vacuum that induces airflow through the soil leading to the volatilization and evaporation of volatile organic components. Apart from extraction wells, the SVE system includes air filters, vacuum blower, air and water separator, silencer, valves, and influent and effluent vapor sampling ports. Optimizing the contamination removal includes arranging the number of wells, their configuration and spacing, blower capacity, and treatment above ground. The list of experts that should be part of the design process is extensive: environmental engineer, health and safety expert, mechanical engineer, regulatory expert, geologist, hydrogeologist, civil engineer, and electrical engineer.
An effective SVE system requires operators to conduct four processes: volatilization, advection, diffusion, and desorption. Volatilization is the central process involved in soil vapor extraction. The reason is simple. Contaminants are volatile chemicals that evaporate from the subsurface using extraction wells. Advection is the process of transporting contaminated substances. SVE system uses air as a medium. Contaminants move around the vadose zone because of negative pressure resulting from vacuums in extraction wells. Negative pressure induces contaminant movement to the wells. Diffusion represents the process of diffusing or spreading out contaminants above the surface. Highly concentrated contaminants spread out toward wells with a low concentration. Finally, desorption is the removal of contaminants from a surface. The vacuum causes the movement of contaminants on the soil particles towards the extraction wells due to a concentration gradient.
Establishing SVE is a medium-term process (from 1 to 3 years) to long-term (throughout building or site occupation for soil vapor extraction). That means that SVE operation is a dynamic process. The parameters can change that require constant monitoring. Monitoring involves equipment verification and optimization (to ensure it works properly). Furthermore, monitoring is necessary for the continuous optimization of contamination removal. Finally, monitoring enables operators to determine when the remediation is complete. The effectiveness of the SVE system depends on the proper operation and monitoring of the process.
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